This invention generally relates to radiation detecting devices, and is specifically concerned with a portable conveyor apparatus for detecting localized concentrations of radioactive materials in the protective garments used in nuclear power facilities.
Conveyor-type devices for detecting radiation in such protective garments are known in the prior art. Generally, these devices comprise a horizontally disposed conveyor belt, and a radiation detector mounted adjacent to the belt for determining whether or not any of the garments fed through the conveyor radiate unacceptably high levels of radioactivity. Some of these prior art devices also include a movable support structure, such as a table on casters, for supporting the conveyor belt and radiation detector and rendering it portable. Such devices are often used to determine whether or not a particular garment that has been worn by a maintenance worker in a nuclear facility still contains radioactive material after being subjected to a decontamination cleaning. In operation, garments that have been subjected to such a decontamination cleaning process are fed through the conveyor belt while being monitored by the radiation detector mounted adjacent to the belt. If the level of the radioactivity of the garment exceeds a selected level, the detector triggers an alarm circuit which notifies the system operator to remove the contaminated article of clothing for further decontamination processing or disposal.
While there are conveyor-type radiation detectors in the prior art which are generally capable of determining whether or not a particular garment is still contaminated with an unacceptably high amount of radioactive material, the applicants have noted a number of shortcomings in these prior art devices. One such shortcoming is the manner in which these devices solve the problem of preventing the radiation alarm circuits from being spuriously actuated by background gamma radiation. This is a serious problem, as these conveyor-type radiation detectors are often operated in the laundry room of a nuclear facility where highly contaminated protective garments and assorted decontamination equipment (i.e., washers, filters, dryers) radiate significant amounts of gamma radiation. To prevent the spurious triggering of the radiation alarm circuitry in these laundry rooms, some of these devices exclusively rely upon a microprocessor which has been programmed to periodically sample the background gamma radiation when no garments are disposed adjacent to the detector, and to subtract the sampled background radiation value from the readings obtained by the radiation detector when garments are passed thereunder. While such exclusive reliance upon "background subtraction" obviates the need for providing thick and heavy lead shielding around the radiation detector to block out such background gamma radiation, it can also cause the device to give inaccurate or false readings since the background gamma radiation in a nuclear facility fluctuates considerably due to the movement of contaminated equipment in or around the vicinity of the device.
To solve the problems associated with exclusive reliance upon "background subtraction," other prior art designs provide thick lead shields around the radiation detectors. While such shields rarely succeed in blocking out all of the background gamma radiation, they at least reduce sole reliance upon "background subtraction." However, in order to obtain accurate readings from the radiation detector of a conveyor-type detection device, the distance between the detector and the garments must be reasonably constant. While many such garments lie substantially flat against the conveyor belt, some garments, such as the "duck feet" worn over the shoes of maintenance personnel project upwardly from the conveyor belt. Some prior art designs provide a means for adjusting the height of the detector so that the distance between the detector and the clothes passed through the conveyor belt can stay substantially the same. However, the applicants have noted that changes in the relative position of the radiation detector and the shielding wall changes the shielding geometry of the detector enough to require an immediate adjustment in the background subtraction if the readings taken by such radiation detectors are to remain accurate. This problem could be minimized by increasing the size of the shield walls. However, the relatively large amount of shielding material needed to substantially surround the radiation detector throughout the amplitude of its movement would add substantial weight to the device as a whole, thereby impairing the portability of the device.
Another problem associated with many of the prior art conveyor-type radiation detector devices results from the fact that the radiation detectors used are often of a single-zone type, thereby making it difficult if not impossible to determine whether radiation emitted by the garment is the result of a single, localized "hot particle," or is the result of a contaminant that is uniformly spread around the garment. This is a serious deficiency, as the applicants have noted that the type of contamination which most commonly necessitates the reprocessing of a particular garment is almost never uniformly disposed throughout the area of the garment, but instead is localized in small (less than 100 cm.sup.2) "hot spots" or as a single, microscopic "hot particle" whose field of radiation, although small, is intense. Such particles present a real contamination control problem since they are small enough to migrate completely through the fabric forming the protective garment and to lodge themselves in intimate contact with the skin of the worker, where their small but intense field of radiation could have adverse affects on the skin. To resolve whether the radiation detected from a particular garment is located in a single spot or spread out over the area of the garment, some conveyor-type detection devices use a bank of multiple scintillation detectors. However, this type of multiple detector arrangement may have blind spots in areas between detectors which could allow a hot particle to escape detection.
A final shortcoming of many prior art conveyor-type detector devices is the fragility of the detectors that they employ. In many of these devices, thin plastic (Mylar.RTM.) windows are used to cover the radiation-sensitive cells whether they be of the scintillation type (gamma sensitive) or gas-flow proportional (beta sensitive) type. While such covers effectively isolate the detector sensors from the ambient atmosphere and perform the important function of preventing lint and debris from accumulating in the sensor cells, they are delicate and subject to breakage. Such breakage necessitates replacement. In scintillation-type detectors, such replacement is expensive. In gas-flow proportional type detectors, such replacement causes four hours or more of downtime if the replacement detector must be completely purged. Some manufacturers have attempted to solve this problem by providing thicker, stronger windows. However, such thick windows seriously attenuate the sensitivity of the detector cells to beta radiation, which in turn seriously comprises the overall sensitivity of the detector.
Clearly, what is needed is a conveyor-type radiation detector apparatus which is capable of accurately and reliably detecting the presence of excessive radioactive contamination on protective garments that have undergone decontamination, but yet which is small and lightweight enough to be easily handled by a single person in a nuclear facility. Ideally, the radiation detector used in such an apparatus should have a plurality of radiation sensitive zones so that the existance of one or more "hot particles" in a particular garment may be at least generally ascertained. The radiation sensitive zones of the detector should be free of blind spots between detector cells so that no localized areas of radioactive contamination go undetected. Finally, the components of such an apparatus should be resistant to breakage, reasonable in cost and capable of continuous operation with a minimum of downtime.